Radio frequency identification (RFID) is a technology that uses radio waves to transfer data from electronic tag (known as RFID tags or RFID inlays). The information is stored electronically in the tag. For reading out the information, an RFID reader transmits an encoded radio signal to interrogate the tag. The RFID tag therefore includes an antenna. The same antenna is also used for encoding an RFID tag, by means of electromagnetic coupling. In the simplest case, an antenna is represented by a planar metallic trace into which electromagnetic energy can be coupled.
An RFID device that includes an antenna is usually referred to as an inlay. In particular, an inlay is an RFID device including a flexible metallic antenna film supported on a flexible substrate, which is connected to a transponder. The transponder is an integrated circuit for deciphering signals sent to the inlay and received by the antenna and also for sending a signal to the antenna, which is then transmitted by the antenna. The inlay antenna may be tuned (i.e. sized) to communicate at a certain target frequency with a transceiver, which is sometimes referred to as the interrogator. The interrogator typically includes an antenna for communication with the RFID inlay. An inlay may be active or passive. An active inlay would include its own power source such as a battery, while a passive inlay would receive its power from an external source such as an interrogator.
In recent years, printing devices have become known that enable dislocating an RFID inlay on a medium such as a sheet of paper, and, at the same time, enable encoding the RFID with the desired information, during the printing process. The encoding is performed by means of electromagnetic coupling, preferably in the reactive near field. For this purpose, an RFID printer/encoder is provided with an electromagnetic coupler arrangement that fits in a cavity of the printer so as to couple electromagnetic power bearing the coding information into the RFID inlay which is located on the medium, while the medium is guided into the printer/encoder along a media path.
Conventionally, two types of technologies are used to encode RFID tags (inlays) in reactive near field, which are, in principle, suitable for being fitted into printer cavities:
Static coupler designs employ a rigid electrical RF (radio-frequency) circuit, such as a transmission line circuit on a PCB (printed circuit board). Since the circuit is rigid and inlay geometries generally have very high variability in form factor, the RF coupling behavior between coupler and inlay will also have a high variability. Hence, for each inlay type RF window profiles can be uniquely identified. An exemplary embodiment of a coupler of this type is described in US Patent Application US 2011/0090054 A1. The coupler arrangement described therein splits an input signal having a target wavelength to a first signal and a second signal, and inverts the second signal (i.e. shifts the second signal by 180° in phase by means of half wavelength transmission line). The first and the second signal are fed to two different conductive patches of the coupler arrangement.
Alternatively, adaptive coupler designs with external control (also called semi-adaptive couplers) are known in the art. In this technology, the coupler structure is split up in several “coupling cells” such as an array. Each cell is individually controlled by some external circuit and software. This means that for each type of inlay regardless of its geometry an adaptation can be performed in that only cells for that particular inlay form factor are activated, in order to achieve an optimum coupling. In the ideal case, optimum coupling regardless of inlay form factor can be achieved by having prior knowledge of which cells to activate. In order to get this information about which cells need to be activated, knowledge of the inlay profile is required. Therefore, a scanning process of unknown inlay geometries is necessary. The information obtained by scanning can be stored in a memory so that each inlay type only needs to be scanned once. Nevertheless, for each new inlay type the process has to be repeated. Therefore, an adaptive coupler of said type is not operable as a standalone component but must be accompanied by a software solution containing the necessary algorithms for the scanning process required for external control of the cells to be activated, possibly involving the whole printer system. Examples of this type of couplers are described in U.S. Pat. No. 7,348,885 and US Patent Application No. 2010/0285746.
As can be seen from the above, it is a drawback of both the above described conventional coupler types that calibrations are needed in order to adapt the coupling arrangement to particular inlay to be encoded, before an actual encoding can be performed. Either (static coupler) positioning of the inlay for optimum coupling must be known, or (adaptive coupler with external control) the inlay profile formation must be obtained and stored.
In the static case, each inlay type has its own unique required positioning, which cannot be changed for a fixed coupler. The described static designs would therefore not be suitable for certain applications where a desired position cannot be achieved. Also, a case may occur wherein due to the geometric relations, the coupling performance is too weak to manage any encoding.
In the externally controlled adaptive case, there is a need for a scanning functionality in order to know the inlay profile. Furthermore, if the resolution of the coupling cell array is not small enough, inlay types may still exist which cannot be encoded and the required resolution for any future inlay type may be hard to determine. The adaptive coupler cannot be used in an “empty state” in real time without information on how the cell array needs to be activated for any given inlay type. Furthermore, it cannot be provided as a stand alone component which is separated independently from the system it belongs to, but must be integrated with the software and also hardware periphery. Also, the resolution may still be to coarse to manage all inlay types.